1. Field of the Invention
The invention relates to improved solid state galvanic-cell sensors for measuring the concentration of gaseous inorganic acid anhydrides, especially sulfur dioxide.
Sulfur and sulfur dioxide (SO.sub.2) are important chemical species in many high temperature systems. The detection and measurement of the presence and concentration of sulfur dioxide and other inorganic sulfur-containing gaseous compounds which may, for example, be generated and released as effluents in the operation of these high-temperature systems has, in recent years, generated considerable interest with the growing concern regarding atmospheric pollution. Concentrations of sulfur dioxide as low as about 1 part per million (ppm) are believed by some to be injurious to plant life while concentrations of 400 to 500 ppm may be fatal to humans. Additionally, in the presence of moisture, SO.sub.2 gas reacts to sulfurous and eventually sulfuric acid causing considerable corrosion problems and the phenomenon known as "acid rain". Over the years, various techniques, including electrochemical techniques have been developed for detecting the presence of SO.sub.2.
Among the earliest of the electrochemical techniques was the bubbling of air believed to be contaminated with sulfur dioxide through deionized water and detecting the presence of the gas through changes in the water's conductivity. The use of different electrolytes and electrode materials has provided improved response time and accuracy. For example, U.S. Pat. No. 3,622,488 to Chand et al discloses the use of a liquid acidic electrolyte such as sulfuric acid with an inert (noble) metal sensing electrode and a solid, counter electrode. The counter electrode comprises an electroactive material capable of being reduced when interconnected with the sensing electrode and placed in contact with the electrolyte. The presence and concentration of sulfur dioxide at the sensing electrode is indicated by the magnitude of current between the sensing and counter electrodes.
H. Dahms in U.S. Pat. No. 3,756,923 describes a system utilizing a thin layer of liquid electrolyte containing silver ions held to the surface of a sensor electrode together with a counter electrode immersed in the liquid. The sample gas is exposed to the liquid electrolyte and the concentration of SO.sub.2 determined by the change in current between the electrodes.
Salzano et al in U.S. Pat. No. 3,718,546 describe a potentiometric determination of sulfur dioxide using an electrochemical cell having a molten, fused salt electrolyte composed of a combination of lithium sulfate (Li.sub.2 SO.sub.4), potassium sulfate (K.sub.2 SO.sub.4) and sodium sulfate (Na.sub.2 SO.sub.4). A reference gas electrode containing a known concentration of sulfur dioxide in air and a sample gas electrode containing an unknown concentration of sulfur dioxide in air are placed in contact with the electrolyte. The difference of electromotive potential between the two electrodes can be related to the concentration of sulfur dioxide in the sample gas.
A solid electrolyte and solid reference electrode may be combined to yield a compact, simple solid state galvanic-cell detector. M. Gauthier et al describe in "Solid-State Detectors for the Potentiometric Determination of Gaseous Oxides", Journal of the Electrochemical Society: SOLID-STATE SCIENCE AND TECHNOLOGY, October 1977 (pp. 1579-1583) two galvanic cells employing potassium and silver sulfates for the measurement of SO.sub.2 concentrations in air. In a subsequent article entitled "Progress in the Development of Solid-State Sulfate Detectors for sulfate Oxide", Journal of the Electrochemical Society: SOLID-STATE SCIENCE AND TECHNOLOGY, February 1981 (pp. 371-378), Gauthier et al disclose yet a third potassium sulfate-silver sulfate solid state galvanic-cell for the detection and measurement of sulfur dioxide, but reveal that the potential measurements of each of these three described Gauthier potassium sulfate based solid state cells drift when used over a period of weeks or even days. These cells are also believed to be the subject of Canadian Pat. No. 1,002,599.
It would be highly desirable to provide a simple, solid state detector for SO.sub.2 and similar inorganic acid anhydrides which detector would have greater potentiometric stability than heretofor achieved. While the more recent Gauthier et al article (1981) describes several other solid state sulfur dioxide detectors, each is mechanically more complicated and difficult to manufacture than the potassium sulfate-silver sulfate cells referred to above.
All of the potassium sulfate galvanic cell sulfur dioxide detectors described by Gauthier et al also operate at temperatures between about 700.degree. and 900.degree. C., apparently to provide sufficient ionic conductivity for acceptable detector response times. It is also highly desirable to provide a solid state detector which provides adequate response time while operating at lower temperatures whereby less expensive and less heat resistent materials may be employed in the construction of the detector and less heating needs to be provided in the measurement of sulfur dioxide in the atmosphere.
Jacob and Rao in an article entitled "A Solid-State Probe for SO.sub.2 /SO.sub.3 Based on Na.sub.2 SO.sub.4 -I Electrolyte", Journal of the Electrochemical Society: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY, pp. 1842-1847 (Vol. 126, No. 11, Nov. 1979) describe the possibility of a sulfur dioxide detecting cell employing sodium sulfate and silver sulfate. It is believed however, that such cell would suffer the drawbacks of the Gauthier et al potassium sulfate-silver sulfate cells in that it would be necessary to operate the sodium sulfate cell at a temperature above about 700.degree. C. and the potentiometric stability of the cell would not be adequate for accurate long term gas surveillance.